The purpose of this research project is to study the neuronal structure of the mammalian spinal cord giving particular attention to the ultrastructural organization and specific synaptic characteristics of the various descending fiber systems. During the past two years the principal investigator has conducted through a grant from the National Science Foundation, research on the corticospinal and corticorubrospinal descending pathways in rodents correlating neuroanatomical and neurophysiological findings so as to confirm and extend what is presently known of spinal mechanisms in mammals. Contrary to the condition in most mammals, in a number of rodents as well as in some primitive mammalian and sub-primate species, the corticospinal system projects primarily to the dorsal horn and appears to subserve a role in the sensory rather than in the motor system. The rubrospinal tract in these species however, projects as in other mammals to the intermediate gray and ventral horn terminating in the same areas as do corticospinal fibers. Recent neurophysiological evidence and light microscope findings suggest that there are monosynaptic corticomotoneuronal and rubromotoneuronal connections with ventral horn motoneurons in the spinal cord of the monkey and possibly the cat. Definitive neuroanatomical data on synaptic morphology can only be obtained in conjunction with the electron microscope. Relevant morphological data is lacking. The specific objective of the proposed project is to examine by both light and electron microscopy corticospinal and rubrospinal projectins to the spinal cord in the cat and the monkey giving particular attention to the synaptic morphology of the normal tissues as well as alterations induced following experimental neuronal degeneration. These findings will extend what is known from previous morphological studies and will correlate the data of recent neurophysiological investigations.